This specification relates to the field of mineral and hydrocarbon recovery, and more particularly to the use of high-molecular weight polyglycolic acid as a primary structural member for a degradable oilfield tool.
It is well known in the art that certain geological formations have hydrocarbons, including oil and natural gas, trapped inside of them that are not efficiently recoverable in their native form. Hydraulic fracturing (“fracking” for short) is a process used to fracture and partially collapse structures so that economic quantities of minerals and hydrocarbons can be recovered. The formation may be divided into zones, which are sequentially isolated, exposed, and fractured. Fracking fluid is driven into the formation, causing additional fractures and permitting hydrocarbons to flow freely out of the formation.
It is also known to create pilot perforations and pump acid through the pilot perforations into the formation, thereby dissolving the formation and allowing the hydrocarbons to migrate to the larger formed fractures or fissure.
To frac multiple zones, untreated zones must be isolated from already treated zones so that hydraulic pressure fractures the new zones instead of merely disrupting the already-fracked zones. There are many known methods for isolating zones, including the use of a frac sleeve, which includes a mechanically-actuated sliding sleeve engaged by a ball seat. A plurality of frac sleeves may be inserted into the well. The frac sleeves may have progressively smaller ball seats. The smallest frac ball is inserted first, passing through all but the last frac sleeve, where it seats. Applied pressure from the surface causes the frac ball to press against the ball seat, which mechanically engages a sliding sleeve. The pressure causes the sleeve to mechanically shift, opening a plurality of frac ports and exposing the formation. High-pressure fracking fluid is injected from the surface, forcing the frac fluid into the formation, and the zone is fracked.
After that zone is fracked, the second-smallest frac ball is pumped into the well bore, and seats in the penultimate sleeve. That zone is fracked, and the process is continued with increasingly larger frac balls, the largest ball being inserted last. After all zones are fracked, the pumpdown back pressure may move frac balls off seat, so that hydrocarbons can flow to the surface. In some cases, it is necessary to mill out the frac ball and ball seat, for example if back pressure is insufficient or if the ball was deformed by the applied pressure.
It is known in the prior art to manufacture frac balls out of carbon, composites, metals, and synthetic materials such as nylon. When the frac ball has fulfilled its purpose, it must either be removed through fluid flow of the well, or it must be destructively drilled out. Baker Hughes is also known to provide a frac ball constructed of a nanocomposite material known as “In-Tallic.” In-Tallic balls are advertised to begin dissolving within 100 hours in a potassium chloride solution.
Another style of frac ball can be pumped to a different style of ball seat, engaging sliding sleeves. The sliding sleeves open as pressure is increased, causing the sleeves to overcome a shearing mechanism, sliding the sleeve open, in turn exposing ports or slots behind the sleeves. This permits the ports or slots to act as a conduit into the formation for hydraulic fracturing, acidizing or stimulating the formation.